The NOx contained in the exhausted gas discharged by power plants, plants, automobiles and such is a substance causative of photochemical smog and acid rain. An exhaust gas denitrating method that employs NH.sub.3 as a reducing agent for selective catalytic reduction has been prevalently used as an effective denitrating method principally in thermal power plants. A titanium oxide (TiO.sub.2) catalyst containing vanadium (V), molybdenum (Mo) or tungsten (W) as an active ingredient is used. Particularly, a catalyst containing vanadium (V) is highly active, difficult to be deteriorated by impurities contained in exhaust gases and effective even at low temperatures. Therefore, such a catalyst is the most currently prevalent denitrating catalyst (JP-A No. 50-128681).
Generally, catalyst elements are formed in the shape of a honeycomb or a plate. Various methods of manufacturing catalyst elements have been developed. A well-known flat catalytic plate is formed by coating and cladding a mesh base member formed by working a thin metal sheet in a metal lath and spraying aluminum over the metal lath, a textile fabric or a nonwoven fabric with a catalyst. This flat catalytic plate is worked to obtain a plate-shaped catalyst element 1 having ribs 2 of a wavelike cross section, and flat sections 3 in an alternate arrangement as shown in FIG. 2. A plurality of such catalyst elements 1 are stacked in layers in a case 4 with the ribs 2 extended in the same direction to construct a catalyst unit 8 (JP-A No. 54-79188 and JPO filing No. 63-324676) as shown in FIG. 43. Since this known catalyst unit 8 causes comparatively small pressure loss and cannot be easily clogged with soot and coal ashes, the catalyst unit 8 is employed prevalently in denitrating apparatuses for denitrating the exhaust gases of boilers for thermal power generation.
The number of power generating installations equipped with gas-turbines or combinations of gas-turbines and waste heat recovery boilers has progressively increased in recent years to cope with peak power demand in summer. Most of such power generating installations are located in the suburbs of cities, and exhaust gas processing apparatus must be highly efficient and compact in view of site condition and pollution control. Under such circumstances, a method of efficiently reducing the NOx content of exhaust gases proposed in JP-A No. 55-152552 employs a catalyst unit 8 constructed by stacking catalyst elements 1 as shown in FIG. 2 so that the respective ribs 2 of the adjacent catalyst elements 1 extend perpendicular to each other, and disposes the catalyst unit 8 with the ribs 2 of the alternate catalyst elements 1 extending perpendicular to the direction of gas flow 6 and with the ribs 2 of the rest of the catalyst elements 1 extending in parallel to the direction of gas flow 6 as shown in FIG. 44.
A catalyst unit 11 proposed in JP-Y.sub.2 No. 52-6673 is formed by working metal laths or a metal sheets to obtain corrugated sheets 9 having successive ridges 10 of a wavelike cross section and not having any flat section as shown in FIG. 46, constructing a support structure by stacking the corrugated sheets 9 so that the respective ridges 10 of the adjacent corrugated sheets 9 extend across each other as shown in FIG. 47, and a catalyst is supported on the support structure to complete the catalyst unit 11. The catalyst unit 8 of FIG. 43 needs the following improvements to construct a high-efficiency, compact exhaust gas processing apparatus. FIG. 48 shows some of gas passages defined by the catalyst elements 1 stacked with the ribs 2 extended in parallel to the direction of gas flow 6. Catalyst units 8 of this type causing very small pressure loss, the exhaust gas processing apparatus employing the catalyst unit 8 of this type requires small power for operation. However, since the flows of gas in the gas passages of the catalyst unit 8 is not very turbulent and the distance of movement of the components of the gas in the gas passages is small, the catalytic reaction rate (overall reaction rate) is small and the catalyst is unable to fully exhibit its activity.
When the catalyst unit 8 is constructed by stacking the catalyst elements 1 so that the ribs 2 extend in parallel to the direction of gas flow 6 as shown in FIG. 43, the rigidity of the catalyst unit with respect to the direction in which the ribs 2 are extended (longitudinal direction) is very large, while the rigidity of the same with respect to the direction perpendicular to the longitudinal direction is small. Therefore there are slight differences in width between the gas passage in the longitudinal direction of the ribs 2 and that perpendicular thereto.
In the catalyst unit 8 shown in FIG. 44, in which the respective ribs 2 of the adjacent catalyst elements 1 are perpendicular to each other, the ribs 2 extending perpendicular to the direction of gas flow 6 exert high gas disturbing effect to promote the component substances of the gas to be subjected to a catalytic reaction. However, those ribs 2 work as barriers against the flow of the gas causing a large pressure loss.
A small degree of freedom of changing draft loss and performance is a problem in the catalyst unit 8 shown in FIG. 44. Since the catalyst unit 8 is constructed by alternately stacking catalyst elements 1 of the same shape, the opening ratio of the catalyst unit 8 does not change and hence draft loss does not decrease significantly even if the pitch of the ribs 2 (the distance between the adjacent ribs) is changed. Furthermore, since the length of the catalyst elements 1 must be equal to the size of the frontage of the catalyst unit 8, it is difficult to change the length of the catalyst elements 1 optionally. Naturally, two types of catalyst elements 1 of different shapes, e.g. those different in the pitch of the ribs 2, may be alternately stacked, but such two types of catalyst elements 1 requires complex manufacturing processes entailing increase in manufacturing costs.
In the catalyst unit 8 shown in FIG. 44, the pitch of the ribs 2 that affect significantly the effect of the catalyst on reaction rate and pressure loss is an important factor. Although the ribs 2 are arranged at equal pitches, the distance between the inlet end of the catalyst unit 8 and the first rib 2 and that between the last rib 2 and the outlet end of the catalyst unit 8 with respect to the direction of gas flow 6 are not particularly specified. Since the catalyst unit 8 shown in FIG. 44 is constructed by stacking the catalyst elements 1 of a given length obtained by cutting a continuous catalytic sheet provided with the ribs 2 at given pitches at given intervals, in some cases, the distance between the end of the catalyst unit 8 and the first rib 2 increases when the amount of the catalyst necessary for catalytic reaction increases, i.e., when the length of the catalyst elements 1 is increased. Consequently, the flat section bends and it is difficult to form a uniform flow passages and it is possible that the end section of the catalyst element bends as shown in FIG. 45 to block the gas passage, lowering the performance of the catalyst unit 8 due to increase in the draft resistance and unbalanced gas flows.
The corrugated catalyst elements 9 of the catalyst unit 11 shown in FIG. 47 do not have any sections corresponding to the flat sections 3 of the catalyst elements 1 shown in FIG. 2. Therefore, when the height of the ridges 10 is substantially equal to that of the ribs 2 of the catalyst elements 1 shown in FIGS. 43 and 44, the ridges 10 of the adjacent corrugated catalyst elements 9 are in contact at a very large number of contact points. Therefore, when the gas flow 6 flows across the section of the cubic catalyst unit 11, the numerous contact points of the ridges 10 cause draft resistance against the gas flow 6, increasing pressure loss.
Accordingly, it is a first object of the present invention to solve problems in the prior art and to provide a catalyst unit capable of enhancing the turbulence of a gas to be processed in the gas passages thereof to suppress the formation of laminar films and of further enhancing catalytic actions.
A second object of the present invention is to solve problems in the prior art and to provide a catalyst unit capable of causing a gas to be processed to diffuse satisfactorily over catalytic surfaces without increasing pressure loss and of enhancing the performance of the catalyst.
A third object of the present invention is to solve problems in the prior art and to achieve exhaust gas purification by using a catalyst unit capable of enhancing the performance of the catalyst by further leveling the flow velocity distribution of a gas to be processed without causing pressure loss in the gas flow.